Reduce static coupling to Controller::has_curr() and Controller::curr()

[sminnee]

A lot of support code (e.g. the error display handler) winds up hooking into Controller::has_curr() and Controller::curr(). This is awkward coupling, and it will make it harder to use SilverStripe code with other controller implementations.

Initially, I've posted this issue as an anchor point so that I can refer to it in TODO comments in code.

[tractorcow]

A lot of the time this is necessary because the current request is only available from the current controller. Maybe we should have SS_HTTPRequest::curr() instead?

[sminnee]

My reason for raising this was to suggest to that we reduce the amount of static coupling. SS_HTTPRequest::curr() would be of limited value.

[sminnee]

For example, we might want to inject something into HTTPOutputHandler that can receive the responses, I guess that it might be better to have some kind of output destination for errors? If nothing else, it would make it easier to test.

Assuming that we move towards PSR-7, which uses immuatable response object, then the injected property might be a writer with an interface like this

interface HttpWriter {
    function write(Psr\Http\Message\ResponseInterface $response);
}

HTTPOutputHandler can then output like this, rather than calling response->output(), and probably don't need to get the Controller response at all...

$this->httpWriter->write($errorResponse);